The sponge in the fluval will serve a dual purpose; mechanical filtration as it will catch detritus and biological, as the nitrifying bacteria will colonise it. Chemical filtration (meaning carbon in this case although there are others) is unneccessary unless you are removing meds or tints from the water (like driftwood stain). I *think* fluvals also offer a space for chemical filtration but I might be wrong there.
Heh, and any backround you wish would be considered good; the fish don't really care that much. They might have an issue with mirrors if they're territorial, but they don't know whether its pictures of rocks or aliens in the back of the tank *grin* I've seen folks use colored aluminum foil (on the outside of course, not in the tank).
aka Cycling Guru and the Ich Slayer
*glares at Terry and QTOFFER*
Card carrying member of FTAS & GCAS.